1. Field of the Invention
The present invention generally relates to a mechanism for moving a projection lens assembly in a projecting optical system to alter projecting magnification, and more particularly, to the improvements in the mechanism for moving the projection lens assembly along a curved path when the projecting magnification is altered.
2. Description of the Related Art
FIG. 1 depicts a projecting optical system employed in a conventional electrophotographic copying apparatus. In FIG. 1, light emitted from a light source 1 is reflected by an original document (not shown) placed on a document platform 2. The light is then applied to a photosensitive drum 9 via a slit member 3, scanning mirrors 4, 5 and 6, a projection lens assembly 7 and a projection mirror 8. In this way, an image on the original document is projected onto the photosensitive drum 9.
FIG. 2 is a diagram indicative of the relationship between an original document P and projected images in an optical system of the type in which the document P is placed in abutment with a reference position PO located on one side of the document platform 2. In such an optical system, the lens assembly 7 is moved along a certain predetermined curve when the projecting magnification is altered. More specifically, the lens assembly 7 is located at the very center 7a between the document P and a projected image Qa at an equal size magnification. At an enlarged size magnification, this lens assembly 7 is drawn towards a position 7b, which is closer to the document P and the reference position PO than the central position 7a is. In contrast, at a reduced size magnification, the lens assembly 7 is drawn towards a position 7c, which is closer to the projected image and farther from the reference position PO than the central position 7a is. By curvedly moving the lens assembly 7 in this way, the projected images Qa, Qb and Qc respectively at the equal size magnification, the enlarged size magnification and the reduced size magnification are caused to correspond to a reference position QO located on one side of the photosensitive drum 9. Upon movement of the lens assembly 7, the projection mirror 8 is moved to correspond to the change in conjugate length.
FIGS. 3 and 4 depict a conventional mechanism for moving the projection lens assembly, which is employed in the above described projecting optical system. This mechanism is comprised of a barrel table 23 for securely supporting a lens barrel 10 of the lens assembly 7 and a movable member 22 which can move in the direction as shown by an arrow A. The barrel table 23 is so held on the movable member 22 as to be slidable in the direction as shown by an arrow B. The lens barrel 10 is fixedly mounted, by means of a belt 12, on a support block 11, which is placed on the barrel table 23. The table 23 is provided at its lower portion with guide pins 13 and 14, which are in mesh with respective openings 22a and 22b formed in the movable member 22 and elongated in the direction of the arrow B. A free end of the guide pin 14 extends downwardly through the elongated opening 22b and is provided with a guide roller 17, which is in contact with a cam surface 15a of a cam plate 15 fixed to the apparatus body. The movable member 22 is screwed to a guide rod 20, which is provided at its peripheral surface with a feed screw 20a and coupled to a stepping motor 21.
When the lens assembly 7 is moved to alter the projecting magnification, the guide rod 20 is rotated by the stepping motor 21 to move the movable member 22 in the direction of the arrow A. Upon such movement, the barrel table 23 moves along the cam surface 15a in the direction of the arrow B, since the guide roller 17 is in contact with the cam surface 15a. Accordingly, the lens assembly 7 moves in both the directions of the arrows A and B.
However, in the foregoing conventional mechanism for moving the projection lens assembly, the guide rod 20 for guiding the lens assembly in the direction of the arrow A is not located on the same plane as the cam plate 15 for restricting the position of the lens assembly in the direction of the arrow B, thus resulting in an increase of the volume occupied by the mechanism. Accordingly, it is difficult to make the projecting optical system thin and the entire apparatus compact.
The U.S. Pat. No. 4,691,957 discloses another mechanism for curvedly moving a projection lens assembly when the projecting magnification is altered. In this mechanism, the lens assembly is moved along a guide shaft extending obliquely with respect to the optical axis. Furthermore, the guide shaft itself is under translation operation. Accordingly, the mechanism as disclosed in this patent is complicated in construction. In addition, since a cam plate for positioning the lens assembly is disposed aside the projection lens assembly, the entire mechanism requires a relatively wide space.